This invention relates to semiconductor photodetectors having Schottky barriers at the electrodes thereof and, more particularly, to an interdigitated photodetector having a non-lattice matched heterojunction surface layer with a band gap greater than that of an underlying light absorbing semiconductor, for inhibiting the entrapment of charge carriers on the semiconductor surface, thereby to prevent tunneling for improved signal quality.
A semiconductor photodetector is responsive to incident radiation by a process of converting photons to electron-hole pairs within a layer of semiconductor material thicker than the radiation penetration depth. The resulting electrons and holes are drawn off as electric current between a pair of electrodes located on one or more surfaces of the semiconductor layer. In one form of a semiconductor photodetector of interest, the electrodes are formed as an interdigitated electrode structure, and are biased by an external source of voltage to draw off the electrons and the holes. The electrodes may be constructed of metal joined to the semiconductor material by either ohmic contacts or Schottky barrier contacts.
Of particular interest herein is the photodetector structure employing the Schottky barrier contacts. The potential barrier of the Schottky construction is desirable for inhibiting the generation of electric current except by photon generated electron-hole pairs. This is useful in the transmission of digital signals optically by radiation such as infra-red and visible light, the optical signals being converted to electric signals by the photodetector. Such transmission may be accomplished by pulses of radiation. The Schottky construction is advantageous in that the foregoing action of the potential barrier inhibits the appearance of noise currents which would increase noise and distort the digital signals.
A problem arises in the foregoing structure wherein elements of the electrodes extend in a parallel array across the surface of the semiconductor material. At the edges of the electrodes, intense potential gradients are present which can cause tunneling. The electron and the hole currents induced by the radiation can be entrapped at the surface of the semiconductor material. The entrapped charge carriers alter the profile of the potential fields at the sites of the Schottky barriers to increase the potential gradient even more, narrowing the potential barriers into the semiconductor surface. In accordance with quantum mechanics, the tunneling of charges through the barriers increases, an effect of the tunneling being the appearance of noise currents and distortion in the reception of the foregoing digital signals. The noise currents may also be accompanied by an increase in the dark current of the photodetector, the dark current being a current which flows even in the absence of incident radiation. The tunneling, therefore, diminishes the advantage of using the Schottky construction in the photodetector.
One aspect of the entrapment of the charge carriers is the fact that the effect of the entrapped carriers, namely, the alteration of the potential field, changes slowly with time. For example, a pulsed infrared signal incident upon the detector would be converted by the altered potential field to an electrical pulse having a relatively sharp leading edge followed by a slow drift in amplitude. The drift is a result of the slow variation in the disturbance of the potential field by the entrapped charge carriers. This effect may be characterized as an undesirable low frequency gain which alters the waveform of an incoming pulse signal train causing intersymbol interference. A further effect is large dark currents and diminished responsivity of the photodetector and its external circuits to incoming radiation signals.
One still further problem arising from the use of prior interdigitated Schottky barrier radiation detectors, which employ gallium-arsenide (GaAs) as the photodetector, is that the detectors are limited to a useable longest wavelength of approximately 0.8 microns. The use of longer wavelength optical signals is desirable due to the lower dispersion of longer wavelength signals within radiation conveying optical fibers and also to evidence of higher inherent reliability of a longer wavelength radiation source, such as a laser. Typically, however, radiation absorbing semiconductor materials, such as InGaAs and InGaAsP, which have a sufficiently small band gap capable of detecting such longer wavelength radiation, are lattice mismatched to those accompanying semiconductor substrates which are currently useful for fabricating integrated circuits such as LSI GaAs MESFET circuits, thereby severely limiting their use. In addition to this lattice mismatch, these smaller band gap semiconductor materials also suffer from the aforedescribed surface state effects which result in a decreased photodetector signal-to-noise ratio.
It has been known to provide a doped region of photodetecting semiconductor material at the surface of a GaAs photodetector to reduce these surface state effects, as is disclosed in copending application Ser. No. 883,187, filed July 8, 1986. While beneficially inhibiting the entrapment of charge carriers, in some applications a small number of charge carriers which are created in the doped region may be attracted to the surface, resulting in the doped region having to be made thinner than may be desired.